An electron gun (i.e. electron beam generation and collimating apparatus) is a component of a variety of different instruments, including but not limited to electron microscopes and electron beam lithography devices. The thermal field emission type and the cold field emission type electron guns are becoming the most commonly used owing to their suitability for medium to high resolution imaging and probe forming applications.
Such an electron gun typically includes a needle pointed electron emitter surrounded by a suppressor or a shielding electrode, an extraction electrode, a post-extraction accelerator, and a post-extraction condenser lens. In a traditional electron gun design, typically only one electrostatic type condenser lens is used, and is positioned after the extraction electrode. The electric field between the electron emitter and the extraction electrode determines the electron trajectory leaving the extractor electrode. From the point of view of the condenser lens, the emitter in effect acts as a virtual source of the electron beam, positioned at a fixed distance behind the emitter surface, with a finite equivalent emission area or source size. This source size is the combined result of the linear magnification of the actual emission area on the emitter surface, and trajectory aberrations induced by the lens actions of the extraction field.
In such a conventional arrangement, the source size cannot be changed with respect to the condenser lens. This is because the voltage difference between the electron emitter and the extraction electrode remains to fixed in order to maintain a stable emission current. As a result, when the electron gun is combined with a probe forming objective lens to form an electron probe for applications such as a scanning electron microscope or an electron beam lithography tool, the choice of the combination of the final probe size and probe current becomes limited. Specifically, final probe size and probe current are coupled and determined by the adjustment of the condenser lens and the probe forming lens. In such a case, only a single curve of probe size vs. probe current may be obtained for each beam limiting objective aperture. This is illustrated in FIG. 1, which plots probe size versus probe current for an example of a single aperture.
In electron guns where a magnetic type condenser lens is used in place of the electrostatic type lens, the magnetic field is either far enough from the emitter so that the magnetic field is insignificant to the emitter and thus functionally equivalent to the electrostatic lens, or the emitter is immersed in the magnetic field. For the immersion case, the condenser lens will not only exert collimating action on the electrons post extraction electrode, but also affect electron trajectory before the extraction electrode, causing changes in the virtual source size and its location. As these collimating and trajectory effects are acted upon simultaneously by one lens and cannot be adjusted independently, the benefits of changeable virtual source size cannot be utilized to expand the adjustable range of the final probe size and current. Moreover, this coupling makes determining the actual probe size and current a complex matter.
Achieving greater flexibility and range in the combination of probe sizes and probe currents available to electron beam equipment could be important for many applications. For example, in semiconductor defect inspections, electric voltage contrast imaging demands a suitable choice of probe current and size.
Accordingly, there is a need in the art for improved designs for electron gun apparatuses and methods of forming electron beams.